Siddhārtha Gautama, better known as Buddha said:
Do not dwell in the past, do not dream of the future, concentrate the mind on the present moment.
He spent good part of his life searching for the "secret" of happiness, and found that it can be achieved by eliminating the causes of suffering. And while achieving happiness and eliminating the pain from us might not be an easy task, we need to understand where happiness roots are.
The root of happiness is inside us, each one of us have to decide whether you want to be happy or not. Happiness is a state of our minds, is how we feel when facing life's facts. As a state of our minds, we should be in control of it, and should be able to be happy just because we have decided to.
Happiness is not something easy to find, specially when we want to achieve way too many goals, and because those are hard to achieve, and because it takes too much time to get to point we want to be and sometimes we never get to it, we start to accumulate frustration. That frustration mines our capability to be happy.
We need to be able to stop frustration. In fact we must stop making our happiness an effect of achieving goals, or accumulate things. Our happiness must rest on the foundation of our will to be happy.
I'm not saying we should stop chasing dreams, that can lead to mediocrity, but certainly should stop relying on things or hitting milestones of our life to be happy. The day we finally understand that the roots of happiness are within us, we will have many more happy days than unhappy ones.